The purpose of this research program is to continue our studies of the dynamic aspects of total body nitrogen metabolism to be studied in young adult humans, elderly people and obese subjects, under various nutritional states. These aspects include estimates of the flux of nitrogen through the metabolic pool and rates of total body protein synthesis and catabolism. We will explore whole body N metabolism with the aid of 15 N-amino acids, given continuously over a 48-60 hour tracer period. The model proposed has already been partially evaluated and we will continue to evaluate it and modify it if necessary in our continuing investigation of the ways in which human body protein metabolism responds adaptively to alterations in the adequacy or level of energy and protein provided by the diet. The overall purpose of these studies is to provide an understanding of the nature of the adaptive responses of human protein metabolism in order to develop better measures of nutritional status and a more rational basis for assessing minimum physiological needs for protein.